In recent years, the application of a secondary battery such as a lithium ion battery is rapidly expanded to a power storage apparatus for storing electric power, which is combined with a new energy system such as a solar battery and wind power generation, a storage battery for automobile, and the like. In the case where many power storage elements such as unit batteries (also referred to as unit cell or cell, in the following description, appropriately referred to as battery cell) are used to generate significant power, a configuration in which a plurality of power storage modules are connected to each other in series is employed. In the power storage module, a plurality of battery cells, e.g., four battery cells are connected to each other in parallel and/or in series, thereby constituting a battery block. Many battery blocks are housed in an exterior enclosure, and thus, the power storage module (referred to also as assembled battery) is configured.
Further, a battery system in which a plurality of power storage modules are connected to each other and a common control apparatus is provided for the plurality of power storage modules has been known. In the battery system, a configuration in which each power storage module includes a module controller and the module controller and the control apparatus communicate with each other via a communication means is employed.
In the case where a plurality of battery cells are used, even when one of the plurality of battery cells reaches the use lower limit voltage at the time of discharge due to the difference of self-discharge between the battery cells and the like, another battery cell does not reach the use lower limit voltage yet in some cases. In the case where the battery cells are charged again in such a state, such a problem that some battery cells are not sufficiently charged, and the performance of the battery cells is not sufficiently delivered occurs.
In order to correct such variability of the plurality of battery cells, an attempt has been made to control the balance between the battery cells. Further, in Patent Literature 1, it is described that many battery cells are divided into a plurality of series cell groups, an inter-cell voltage balance correction circuit is provided in each cell group, and an inter-group voltage balance correction circuit is provided. The inter-group voltage balance correction circuit is configured to cause AC (alternating-current) coupling formed by using a transformer coil and a switching circuit to perform balance correction on the series voltage of each cell group.
The inter-group voltage balance correction circuit described in Patent Literature 1 can be applied for correcting the balance of the battery group of the power storage module. However, although a coil is connected for each cell group, the coil is configured to be wound around a common magnetic core. Therefore, in the case where it is connected to the plurality of power storage modules housed in separate enclosures, it needs to house the coil and the magnetic core in separated enclosures. Because star-like wiring in which the plurality of power storage modules are connected to this different transformer apparatus is performed, there is a problem of complicated connection in the case where the number of power storage modules is increased.
Further, because uniform voltage is obtained by controlling the switching circuit to perform a turning on/off operation in the same phase, it is difficult to control the switching operation separately for each cell group. Therefore, there has been such a problem that it is difficult to transmit electric power from a particular cell group having high voltage to a particular cell group having low voltage. Further, in the case where a common coil and switching element are provided on the secondary side of the transformer, total voltage of series circuits of a plurality of cell groups is applied to this switching element, and there has been such a problem that it is difficult to ensure the pressure resistance of the switching element.
The applicant has proposed a power storage apparatus in which it does not need to configure a transformer as an apparatus different from a power storage module and the pressure resistance of a switching element can be easily ensured as described in Patent Literature 2. In the power storage apparatus described in Patent Literature 2, the modules are well-balanced by providing a transformer and a switching element for each module and controlling turning on/off of the switching element to cause charges to move from the module having high voltage to the module having low voltage. Specifically, the difference between the amount of charge on the side of high voltage and the amount of charge on the side of low voltage side is eliminated separately in several times.